Transition strategy for the production of sticky polymers

Details Transition strategy for the production of sticky polymers Transition strategy for the production of sticky polymers

2000-10-24

2002-05-14

Teskin, Fred (Department: 1713)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C526S084000, C526S088000, C526S212000, C526S901000

Reexamination Certificate

active

06388027

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to transitioning from one sticky polymer (or elastomer) to another. More particularly, the invention relates to a process from transitioning from a first sticky polymer to a second sticky polymer such as EPDMs in a gas phase fluidized bed reactor.
BACKGROUND OF THE INVENTION
The goal of any commercial production facility is to maximize the amount of aim grade product. Most modern continuous polyolefin facilities produce a wide range of products (homo- and co-polymers of alpha olefins) where each product requires different reaction process conditions. Product wheels and transition strategies are developed to minimize the amount of off-grade material made during the transition between different products and to minimize the time between such products. Typical strategies for continuous processes for these polymers can include a reduction in process inventory, reducing the production rate during a transition, changing the reactor conditions as rapidly as possible, and using other sophisticated control parameter trajectories that deliberately undershoot or overshoot the desired final steady state reaction conditions.
There is an on-going need to develop a transition process for sticky polymers.
SUMMARY OF THE INVENTION
There is provided a process for transitioning from a first sticky polymer to a second sticky polymer in a gas phase fluidized bed polymerization, which process comprises the steps of:
(a) terminating the feed of catalyst to a reactor, thereby allowing reaction rate to decrease;
(b) terminating the polymerization using a reversible catalyst kill agent;
(c) passivating the polymer with a gel inhibitor;
(d) stopping polymer transfer from the reactor to the post-reaction purging and polishing equipment, thereby recovering aim-grade product from the post reaction equipment;
(e) flow and pressure purging to remove kill agent
(f) feeding cocatalyst and optional promoters to establish concentrations of these components;
(g) establishing reaction conditions for the second sticky polymer;
(h) re-initiating catalyst feed; and
(i) re-initiating fluidization aid, cocatalyst, optional promoter and diene feeds.
DETAILED DESCRIPTION OF THE INVENTION
It has been discovered that the transition of a fluidized bed gas phase reaction for a sticky polymer is more complicated compared to transitions for other polyolefin products utilizing alpha olefins as the monomer(s). For example, ethylene-propylene-diene elastomers (EPDMs) are inherently more sticky than polyethylene or polypropylene homopolymers and copolymers. Thus, they are more susceptible to defluidization or agglomeration with changes in process conditions. For instance, the more rapidly the change occurs the more likely it is that defluidization and/or agglomeration will take place.
Therefore, decreases in sticky polymer crystallinity attributable to a decreasing ethylene content make it necessary to change the monomer concentration slowly to minimize agglomeration of the product.
Also, the concentration of fluidization aid which controls the sticky polymer (e.g., EPR product) particle size must be adjusted as the product crystallinity changes. Unlike gas phase monomer concentrations where a change in monomer concentration can be easily predicted by mass balance, changes in fluidization aid concentration exhibit a large lag. The lag is caused by changes in reactor inventory which is not accounted for by monitoring the increase in fluidization aid content in the product. It is believed that fluidization aid particles deposit on the walls of the process equipment and piping and which can subsequently be eroded away from these areas in addition to adhering to the sticky EPR product. This erosion-deposition process is not well understood but it creates a lag in response to an instantaneous change in fluidization aid flow rate. This ill-defined lag creates problems using strategies which assume that there is no accumulation within the reactor. Lags of more than a day have been observed in response to a change in fluidization aid flow rate.
Transitioning between products with widely varying fluidized bulk densities is tricky in a fluidized bed reactor because the bed tends to compact more for one product versus another. The variation in bed weight caused by the varying bed level also changes the reactor residence time which changes catalyst activity and influences final product properties such as the molecular weigh or gel content of the product. Adjusting the bed level on the fly (rapidly) can result in near reactor shutdowns in a commercial facility.
The transition strategy practiced prior to this invention in the production of EPRs (and EPDMs) was to gradually change the gas phase composition in the reactor and the fluidization aid content of the product to the new conditions while the reactor continued to make product. During the transition, much of the product produced at the transitioning process conditions did not meet specification for aim-grade material and must be sold at a considerable loss in revenue.
In the present invention, on-line transitions in gas-phase EPDM reactions (as well as for other sticky polymers) need to be done gradually (1) to avoid fluidization and agglomeration problems in the bed and (2) to minimize swings in production rate due to changes in reactor residence time that can occur as the fluidized bulk density of the product changes.
As the EPDM product composition changes during, transitioning, the stickiness of the product changes. Additional fluidization aid such as carbon black is added to prevent the bed from defluidizing with the sticky polymer products. (Fluidization aids are also described as inert particulate materials and disclosed in U.S. Pat. Nos. 4,994,534 and 5,304,588.) If too little fluidization aid is present, the bed may agglomerate and defluidize. If the fluidization aid content is too high, the excess fluidization aid will foul the process equipment, particularly the cycle gas coolers and gas distributor plate. Depending on the transition rate, the polymer properties can change faster than the fluidization aid content in the reactor. Lengthening the transition time to provide the desired fluidization aid content for the product being currently being produced increases the production of off-grade material. Table 1 shows the typical process conditions required to make four commercial EPR products.
MEGA-
MEGA-
MEGA-
MEGA-
7211
6322
7265
9315
Process Variable
Ethylene Partial Pressure
80
70
80
70
psia
C3/C2 Mole Ratio
1.0
2.75
1.48
2.3
H2/C2 Mole Ratio
0.02
0.021
0.048
0.013
Carbon Content - phr
12
23
20
20
Product Properties
C2 Content, wt %
78
65
67
64.5
C3 Content, wt %
21
33
29
32
ENB Content, wt %
1
2
4
3.5
Mooney Viscosity at
70
80
80
102
125° C., 1 + 4
Once the EPR product leaves the reactor, it is usually necessary to perform other operations before the product is ready for shipment to a user. Stabilizer addition, monomer stripping, catalyst de-ashing, and packaging typically occur in post reaction processes as disclosed in U.S. Pat. No. 5,548,040 and U.S. Ser. No. 09/098,479. In a commercial facility, fluidized beds are used to heat the product, stabilize it, quench the excess aluminum alkyl with alcohol, and purge residual monomer from the product before it is sent on to packaging.
The process prior to this invention involved diluting the first (or “old”) product with the second (or “new”) product which further increasing the amount of off-grade material produced. The back-mixed nature of the fluidized bed purger or polisher required additional product residence times in the purger polisher before the steady state composition was representative of the product being discharged from the reactor. The overall existing reaction strategy prior to this invention wasted raw materials, produced excessive amounts of off-grade product, took a substantial time to convert from one product grade to another thereby resulting in less than optimal unit efficiency.
The preferred process for transitioning between two different sticky polymers (i.e

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